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2     <html>
3     <head>
4     <title>JSR 166 Snapshot Introduction.</title>
5     </head>
7     <body bgcolor="#ffffee" vlink="#0000aa" link="#cc0000">
8     <h1>JSR 166 Snapshot Introduction.</h1>
10     by <a href="http://gee.cs.oswego.edu/dl">Doug Lea</a>
11     <p>
13     To join a mailing list discussing this JSR, go to:
14     <A HREF="http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest"> http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest</A> .
16     <p>
17     <em>
18     Disclaimer - This prototype is experimental code developed as part of
19 dl 1.3 JCP JSR166 and made available to the developer community for use
20 tim 1.1 as-is. It is not a supported product. Use it at your own risk. The
21     specification, language and implementation are subject to change as a
22     result of your feedback. Because these features have not yet been
23     approved for addition to the Java language, there is no schedule for
24     their inclusion in a product.
25     </em>
27 dl 1.3 <p> Package java.util.concurrent contains utility classes commonly
28     useful in concurrent programming. Like package java.util, it includes
29     a few small standardized extensible frameworks, as well as some
30     classes that provide useful functionality and are otherwise tedious or
31     difficult to implement. JSR166 also includes a few changes and
32     additions in packages outside of java.util.concurrent: java.lang, to
33     address uncaught exceptions, and java.util to better integrate with
34     collections. Since the target release is JDK1.5, many APIs use
35     generics to parameterize on types. Here are brief descriptions of the
36     main components.
37 tim 1.1
38 dl 1.3 <h2>Executors</h2>
40     {@link java.util.concurrent.Executor} is a simple standardized
41     interface for defining custom thread-like subsystems, including thread
42     pools, asynch-IO, and lightweight task frameworks. Depending on which
43     concrete Executor class is being used, tasks may execute in a newly
44     created thread, an existing task-execution thread, or the thread
45     calling <tt>execute()</tt>, and may execute sequentially or
46     concurrently. Executors also standardize ways of calling threads that
47     compute functions returning results, via a {@link
48     java.util.concurrent.Future}. This is supported in part by defining
49     interface {@link java.util.concurrent.Callable}, the argument/result
50     analog of Runnable.
52     <p> {@link java.util.concurrent.ExecutorService} provides a more
53     complete framework for executing Runnables. An ExecutorService
54     manages queueing and scheduling of tasks, and allows controlled
55     shutdown. The two primary implementations of ExecutorService are
56     {@link java.util.concurrent.ThreadPoolExecutor}, a highly tunable and
57     flexible thread pool and {@link
58     java.util.concurrent.ScheduledExecutor}, which adds support for
59     delayed and periodic task execution. These, and other Executors can
60 dholmes 1.5 be used in conjunction with a {@link java.util.concurrent.FutureTask}
61     to asynchronously
62 dl 1.3 start a potentially long-running computation and query the FutureTask
63     to determine if its execution has completed, or cancel it.
65     <p> The {@link java.util.concurrent.Executors} class provides factory
66     methods for the most common kinds and styles of Executors, as well as
67     a few utilities methods for using them.
68 tim 1.1
69     <h2>Queues</h2>
71 dl 1.3 A basic (nonblocking) {@link java.util.Queue} interface extending
72     java.util.Collection is introduced into java.util. Existing class
73     java.util.LinkedList is adapted to support Queue, and a new
74 dl 1.6 non-thread-safe {@link java.util.PriorityQueue} is added. The
75     java.util.concurrent {@link
76     java.util.concurrent.ConcurrentLinkedQueue} class supplies an
77     efficient sclable thread-safe non-blocking FIFO queue, and {@link
78     java.util.concurrent.ConcurrentLinkedStack} provides a similar
79     non-blocking LIFO stack.
80 tim 1.1
81     <p> Five implementations in java.util.concurrent support the extended
82 dl 1.3 {@link java.util.concurrent.BlockingQueue} interface, that defines
83     blocking versions of put and take: {@link
84     java.util.concurrent.LinkedBlockingQueue}, {@link
85     java.util.concurrent.ArrayBlockingQueue}, {@link
86     java.util.concurrent.SynchronousQueue}, {@link
87 dholmes 1.5 java.util.concurrent.PriorityBlockingQueue}, and
88     {@link java.util.concurrent.DelayQueue}.
89 tim 1.1
91     <h2>Locks</h2>
93 dl 1.3 The {@link java.util.concurrent.Lock} interface supports locking
94     disciplines that differ in semantics (reentrant, fair, etc), and that
95     can be used in non-block-structured contexts including hand-over-hand
96     and lock reordering algorithms. This flexibility comes at the price of
97     more awkward syntax. Implementations include {@link
98     java.util.concurrent.ReentrantLock} and {@link
99     java.util.concurrent.FairReentrantLock}.
101     <p> The {@link java.util.concurrent.Locks} class additionally supports
102     some common trylock-designs using builtin locks.
104     <p> The {@link java.util.concurrent.ReadWriteLock} interface similarly
105     defines locks that may be shared among readers but are exclusive to
106     writers. Only a single implementation, {@link
107     java.util.concurrent.ReentrantReadWriteLock}, is provided, since it
108 tim 1.1 covers all standard usage contexts. But programmers may create their
109     own implementations to cover nonstandard requirements.
111     <h2>Conditions</h2>
113 dl 1.3 The {@link java.util.concurrent.Condition} interface describes the
114     kinds of condition variables associated with monitors in other
115     concurrent languages, as well as pthreads-style condvars. Their
116     support reduces the need for tricky and/or inefficient solutions to
117     many classic concurrent problems. To avoid compatibility problems,
118     the names of Condition methods are different than Object versions.
119 tim 1.1
120 dl 1.3 <h2>Atomics</h2>
121 tim 1.1
122 dl 1.2 The atomic subpackage includes a small library of classes, including
123 dl 1.3 AtomicInteger, AtomicLong, and AtomicReference that support
124     compareAndSet (CAS) and related atomic operations.
125 tim 1.1
126     <h2>Timing</h2>
128 dl 1.3 The {@link java.util.concurrent.TimeUnit} class provides multiple
129     granularities (including nanoseconds) for both accessing time and
130     performing time-out based operations.
131 tim 1.1
132     <h2>Synchronizers</h2>
134     Five classes aid common special-purpose synchronization idioms.
135 dl 1.3 {@link java.util.concurrent.Semaphore} and {@link
136     java.util.concurrent.FairSemaphore} are classic concurrency tools.
137     {@link java.util.concurrent.CountDownLatch} is very simple yet very
138     common utility for blocking until a single signal, event, or condition
139 dholmes 1.5 holds. A {@link java.util.concurrent.CyclicBarrier} is a resettable multiway
140 dl 1.3 synchronization point common in some styles of parallel
141     programming. An {@link java.util.concurrent.Exchanger} allows two
142 dl 1.2 threads to exchange objects at a rendezvous point.
143 tim 1.1
144     <h2>Concurrent Collections</h2>
146 dl 1.6 Besides Queues, this package supplies a few Collection implementations
147     designed for use in multithreaded contexts: {@link
148 dl 1.3 java.util.concurrent.ConcurrentHashMap}, {@link
149     java.util.concurrent.CopyOnWriteArrayList}, and {@link
150     java.util.concurrent.CopyOnWriteArraySet}.
152 dl 1.6 <p>The "Concurrent" prefix for classes is a shorthand
153     indicating several differences from similar "synchronized"
154     classes. For example <tt>java.util.Hashtable</tt> and
155     <tt>Collections.synchronizedMap(new HashMap())</tt> are
156     synchronized. But {@link
157     java.util.concurrent.ConcurrentHashMap} is "concurrent".
158     A concurrent collection (among other kinds of classes) is
159     thread-safe, but not governed by a single exclusion lock. So, in the
160     particular case of ConcurrentHashMap, it safely permits any number of
161     concurrent reads as well as a tunable number of concurrent writes.
162     There may still be a role for "synchronized" classes in some
163     multithreaded programs -- they can sometimes be useful when you need
164     to prevent ALL access to a collection via a single lock, at the
165     expense of much poor scalability. In all other cases, "concurrent"
166     versions are normally preferable.
168 dl 1.3 <p> Most concurrent Collection implementations (including most Queues)
169 dl 1.6 also differ from the usual java.util conventions in that their Iterators
170 dl 1.3 provide <em>weakly consistent</em> rather than fast-fail traversal. A
171     weakly consistent iterator is thread-safe, but does not necessarily
172     freeze the collection while iterating, so it may (or may not) reflect
173     any updates since the iterator was created.
174 tim 1.1
175     <h2>Uncaught Exception Handlers</h2>
177 dl 1.3 The java.lang.Thread class is modified to allow per-thread
178 tim 1.1 installation of handlers for uncaught exceptions. Ths optionally
179     disassociates these handlers from ThreadGroups, which has proven to be
180 dl 1.3 too inflexible in many multithreaded programs. (Note that the
181     combination of features in JSR166 make ThreadGroups even less likely
182     to be used in most programs. Perhaps they will eventually be
183     deprecated.)
184 dl 1.2
185 dl 1.3 <p> Additionally, java.lang.ThreadLocal now supports a means to remove
186     a ThreadLocal, which is needed in some thread-pool and worker-thread
187 dl 1.2 designs.
188 tim 1.1
189     <hr>
190     <address><A HREF="http://gee.cs.oswego.edu/dl">Doug Lea</A></address>
191     </body>
192     </html>

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